This is the beginners area.... but a quick primer is needed on bus technology and hard drive transfer rates:

Keep in mind that most, but not all, bus lines are shared. For example. USB has about 480 mb/s FOR EACH EVERY DEVICE IN TOTAL. So if you use 2 hard drives, they will both cap out the entire bandwidth. Note that is 480 mb NOT MB. So only about 60 megaBYTES per second OF TOTAL BUS. So if you have two drives, both will run at 30 MB per second. 3? 20 MB.

Most people assume that gigabit networking = 1 gigabyte per second. This is false. A byte is 8 bits. So a gigabit network can transfer, at most, 125 MEGABYTES per second. In practice, it is even less. Assume about 100.

Hard drives have about 50 - 60 MB/s transfer rates. MegaBYTE. They may say ATA 150 or whatever but trust me, they cap out at 40 - 60 megaBYTES per second. Of course if you are running SATA, this will likely not be a problem. I have transferred 500+ megaBYTES per second with a RAID6 array. Then you start to hit network bottlenecks.... 1 GigaBIT = only 125 megaBYTEs per second (theoretical).

WoW needs to access a ton of data when you load a new instance. It needs it quickly and there is a good amount of it to load. To a lesser extent, the CPU is also involved as is the GPU to render the scene. It would make sense to split the drives up if possible but the TOTAL bottomline is that playing with even 5 WoWs on a single hard drive is perfectly doable. It would just be faster to fan them out to either an ultra fast hard drive (solid state has better access times but about the same transfer rates) or a RAM based drive (but you lose data if power goes out) or multiple hard drives (more ideal but more expensive and more points of failure).

But ideally, get 1 drive per instance and put them on a SATA bus, not IDE (ATA) or USB. Failing to use the proper bus will actually not improve load times as it has to go through all of these bottlenecks first.

SATA 1.5 (1500 megabits/sec) gives about 120 megaBYTES per second of transfer potential. That is more than any single commercially available rotating disk hard drive out there today. SATA is NOT shared like USB, IDE, even PCI and PCI-E are.

Hope that helps some.